Python+selenium自动化测试：报错：TypeError: 'WebElement' object is not iterable

代码：

# -*- coding: utf-8 -*-
from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.common.keys import Keys
from selenium.webdriver.support.ui import Select
from selenium.common.exceptions import NoSuchElementException
from selenium.common.exceptions import NoAlertPresentException
import unittest, time, re

class KeywordsAnalyse(unittest.TestCase):
    driver=webdriver.Chrome("E:\chromedriver_win32\chromedriver.exe")
    driver.get("https://。。。。。。/oldriver/login")

    driver.find_element_by_id("username").clear()
    driver.find_element_by_id("username").send_keys("")
    driver.find_element_by_id("password").clear()
    driver.find_element_by_id("password").send_keys("000000")
    driver.find_element_by_xpath(".//*[@id='loginForm']/input").click()
    driver.find_element_by_xpath(".//*[@id='side-menu']/li[4]/a").click()
    driver.find_element_by_xpath(".//*[@id='keywords']").send_keys("fidget spinner")

    select=Select(driver.find_element_by_id("category"))
    all_options=driver.find_element_by_id("category")
    for option in all_options:
        print("")
    driver.find_element_by_id("btnSubmit").click()


报道错误：TypeError: 'WebElement' object is not iterable

修改：

  all_options=driver.find_element_by_id("category")改为

  all_options=driver.find_elements_by_id("category")